Apparatus and method for vehicle emissions control

ABSTRACT

An apparatus and method for vehicle emissions control. More particularly, an apparatus and method for insuring that the temperature of a vehicle&#39;s exhaust gas stream entering the underfloor catalytic converter during engine operation does not exceed the temperature at which the capability of the multi-functional catalyst in the converter to absorb NO x  starts to fall off. A first temperature sensor is provided in the exhaust outlet leading from the exhaust manifold of the engine and a second temperature sensor is provided in the catalytic converter. The catalytic converter contains a multi-functional catalyst such as a three-way conversion catalyst and a NO x  trap as well as a NO x  sensor at its outlet for sensing when NO x  breakthrough is occurring. The sensors transmit their readings to a controller which transmits signals to a proportional valve located in the exhaust outlet downstream of the first temperature sensor. In response to the signals received from the controller, the proportional valve divides the exhaust gas stream received from the engine exhaust manifold into a first stream in communication with the catalytic converter and a second stream in communication with a heat exchanger. Any stream exiting the heat exchanger is then blended with the non-heat exchanged stream and the blended stream is passed into the catalytic converter. The amount of the exhaust gas stream in the first conduit will vary from that in the second conduit such that the temperature entering the catalytic converter is maintained in the range of about 300 to about 600° C. during the lean mode operation of the engine and further such that the multi-functional catalyst is periodically purged of NO x  and/or SO x .

FIELD OF THE INVENTION

[0001] This invention relates to an apparatus and method for vehicleemissions control and more particularly, to an apparatus and method forcontrolling the temperature of the exhaust gas stream exiting theexhaust manifold and entering an underfloor catalytic convertercontaining a multi-functional catalyst, e.g., a three-way conversioncatalyst and a nitrogen oxides (“NO_(x)”) trap.

BACKGROUND OF THE INVENTION

[0002] Conventional lean-burn engine control systems include an air/fuelcontroller that delivers fuel to the engine intake manifold proportionalto measured air mass to maintain a desire air/fuel ratio, lean ofstoichiometric. Emissions of nitrogen oxides (“NO_(x)”) from lean-burnengines (described below) must be reduced in order to meet emissionregulation standards. Conventional three-way conversion (“TWC”)automotive catalysts are suitable for abating NO, carbon monoxide (“CO”)and hydrocarbon (“HC”) pollutants in the exhaust of engines operated ator near stoichiometric air/fuel conditions. The precise proportion ofair to fuel that results in stoichiometric conditions varies with therelative proportions of carbon and hydrogen in the fuel. An air-to-fuel(“A/F”) ratio of 14.65:1 (weight of air to weight of fuel) is thestoichiometric ratio corresponding to the combustion of a hydrocarbonfuel, such as gasoline, with an average formula CHI_(1.88). The symbol λis thus used to represent the result of dividing a particular A/F ratioby the stoichiometric A/F ratio for a given fuel, so that λ=1 is astoichiometric mixture, λ>1 is a fuel-lean mixture and λ<1 is afuel-rich mixture.

[0003] Engines, especially gasoline-fueled engines to be used forpassenger automobiles and the like, are now designed to operate underlean conditions as a fuel economy measure. Such engines are referred toas “lean-burn engines”. That is, the ratio of air to fuel in thecombustion mixtures supplied to such engines is maintained considerablyabove the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of18:1) so that the resulting exhaust gases are “lean”, i.e., the exhaustgases are relatively high in oxygen content.

[0004] Although lean-burn engines provide enhanced fuel economy, theyhave the disadvantage that conventional TWC catalysts are not effectivefor reducing NO_(x) emissions from such engines because of excessiveoxygen in the exhaust. The prior art discloses attempts to overcome thisproblem by operating lean-burn engines with brief periods of fuel-richoperation. (Engines which operate in this fashion are sometimes referredto as “partial lean-burn engines”.)

[0005] The typical TWC catalyst provided in the exhaust passage as a“close-coupled” catalytic converter does not convert the NO_(x) producedwhen the engine is running lean, i.e., when λ>1. In order to reduce theNO_(x) emission to the atmosphere, it is known to use an underfloorcatalytic converter located downstream of the medium-coupled orclose-coupled catalytic converter. “Close-coupled” catalytic convertersare known in the prior art and are generally defined as located in ornear the engine compartment, typically less than one foot, moretypically less than six inches from, and preferably immediately adjacentto, i.e., attached directly to, the outlet of the exhaust manifold.“Underfloor” catalytic converters are also known in the prior art andare located (downstream of any close-coupled catalysts) under the floorof the vehicle adjacent to or in combination with the vehicle's muffler.

[0006] It is known to treat the exhaust of such engines with anunderfloor catalytic converter containing a multi-functional catalyst,e.g., a TWC catalyst NO_(x) trap which stores NO_(x) during periods oflean (oxygen-rich) operation, and releases the stored NO_(x) during therich (fuel-rich) periods of operation. A typical NO_(x) trap utilizesalkali metal or alkaline earth metal oxides in combination with theprecious metal catalyst component in order to store or occlude NO_(x)under lean operating conditions. The mechanism for NO_(x) storage isbelieved to involve the oxidation of NO to NO₂ over the precious metalcomponent of the TWC catalyst followed by the subsequent formation of anitrate complex with the alkali metal or alkaline earth metal oxide.Under engine operation rich of stoichiometric (λ<1), the nitratecomplexes are thermodynamically unstable, and the stored NO_(x) isreleased and catalytically reduced by the excess of CO, HCs and H₂ inthe exhaust. Periodically, the lean-burn engine is switched to arelatively rich air/fuel ratio to purge the NO_(x) trap.

[0007] It is known that exposure of the NO_(x) trap to excessivetemperatures, e.g. 750° C. and higher, during the operation of theengine will result in a significant diminution of the capability of theNO_(x) trap to absorb the NO_(x) in the exhaust gas stream Therefore, itwould be desirable to provide some means of insuring that thetemperature of the exhaust gas stream entering the underfloor catalyticconverter containing the NO_(x) trap during the operation of the enginedoes not exceed the temperature at which the capability ofthe trap toabsorb the NO_(x) in the exhaust gas stream starts to fall off.

[0008] Lean-burn engines are designed for fuel economy. In such engines,operations alternate depending on speed and load. At the lean (λ>1)condition, the NO_(x) trap in the underfloor catalytic converter absorbsNO_(x), then a fuel-rich (λ<1) spike is applied which results in NO_(x)desorption from the trap and catalytic reduction of the NO_(x) to N₂,then a lean condition occurs followed by a rich spike, etc. Richconditions are required from time to time at higher speeds and loads inorder to maintain the temperature ofthe exhaust gas flowing into theunderfloor catalytic converter at a temperature below that which wouldresult in deterioration of the NO_(x) trap. At stoichiometric or richerthan stoichiometric conditions, i.e. λ≦1, the multi-functional catalystin the underfloor catalytic converter has the capability of reducing theNO_(x) to N₂without absorption of the NO_(x) by the trap.

[0009] Typically, the lean-bum engine is periodically switched to arelatively rich air/fuel ratio to purge the NO_(x) trap. The NO_(x) trapmust be exposed to minimum threshold temperatures at specific enginespeeds and loads before it will perform efficiently and accordingly, aminimum exhaust temperature must be established before a lean-burn modeof engine operation is established. There are also upper or maximumtemperatures within certain speeds and loads, above which the trap willcease operating effectively for trapping NO_(x). At such highertemperatures with specific speeds and loads, the engine operation willswitch from lean to stoichiometric (or rich) conditions. The catalyst inthe underfloor catalytic converter will act as a TWC catalyst such thatNO_(x), CO and HCs are effectively removed. Since fuel economy issignificantly improved by operating the engine at lean conditions, anapparatus and method are required for sensing the upper temperaturelimit of the lean operation, thus permitting the temperature of theexhaust gas stream entering the NO_(x) trap to be lowered, therebyallowing the lean operation of the engine to be extended to cover higherspeeds and loads which would otherwise be required to occur atstoichiometric conditions. Such extension ofthe lean operation oftheengine would result in dramatically improved fuel savings, whileconcurrently lowering the formation of the greenhouse gas CO₂.

[0010] It is also known that at certain higher speeds and loads, thetemperature of the exhaust gas stream entering the NO_(x) trap mayexceed the temperatures at which the tap starts to deteriorate. At suchextreme conditions of speed and load, the exhaust gas temperature isusually lowered by using fuel enrichment (λ<1) in order to preventdeterioration of the trap. This practice has a severe adverse impact onfuel economy and also defeats the purpose of fuel savings associatedwith the lean burn engines. Accordingly, an apparatus and method arerequired for sensing such upper temperature limits in order to therebylower the temperature ofthe exhaust gas stream entering the trap to atemperature below that which is known to cause deterioration of thetrap, i.e., the temperature is thereby lowered to about 750° C. orlower.

[0011] It is further known that over time, NO_(x) breakthrough occurswhen the trap becomes saturated with NO_(x) and/or sulfur oxides(“SO_(x)”) and must be purged in order to be able to continue toeffectively trap the NO_(x). In order to purge the SO_(x), the NO_(x)trap must be exposed to an exhaust gas stream having a temperature inthe range of about 600° C. to about 750° C. Since the temperature andtime required for purging SO_(x) from the trap is close to that at whichthe trap suffers degradation, an apparatus and method is required forsensing NO_(x) breakthrough and permitting exposure of the trap to thehigher temperature exhaust stream during purging to remove SO_(x).

OBJECTS OF THE INVENTION

[0012] It is an object of the present invention to provide an apparatusand a method for controlling the temperature of the exhaust gas enteringthe underfloor catalytic converter containing a TWC catalyst/NO_(x) trapsuch that the NO_(x) trap will not be exposed to excessively hightemperatures thereby causing a deterioration of the NO_(x) trap duringlean conditions of engine operation.

[0013] It is a further object of the invention to cause the lean-burnmode of operation of the engine to be extended cover higher engineoperation temperatures (i.e., higher speeds and loads) than wouldusually be possible. It is the common practice, at the temperatureswhere the NO_(x) trap become ineffective, to switch the engine operationfrom lean to stoichiometric, i.e., λ>1 to λ=1. This is necessary inorder to facilitate the removal of the gaseous pollutants by the TWCfunction of the catalyst in the underfloor catalytic converter. Theextended lean-burn mode of engine operation thereby improves fueleconomy.

[0014] It is an additional object of the invention to provide anapparatus and a method which, at engine conditions requiring highsspeeds and loads, result in a cooling of the exhaust gas to therebyminimize the necessity of using fuel enrichment which would otherwise berequired to effect such cooling in order to minimize deterioration ofthe NO_(x) trap. This additional object also results in an improvementin fuel economy.

[0015] It is yet a further object of the invention to provide anapparatus and a method for purging the NO_(x) trap of NO_(x) and/orSO_(x) without concurrent deterioration of the trap.

[0016] These and other objects have been achieved by the invention whichis described in detail below.

SUMMARY OF THE INVENTION

[0017] In accordance with the present invention, an underfloor catalyticconverter containing a multi-functional catalyst, e.g., a TWCcatalyst/NO_(x) trap, is provided in fluid communication with the outletof the exhaust manifold of the engine. A first temperature sensor isprovided in the exhaust outlet and a second temperature sensor isprovided in the catalytic converter; (preferably near the inlet of theconverter). A NO_(x) sensor is located at the outlet of the converter inorder to sense NO_(x) breakthrough, i.e., determine when the trap mustbe purged to remove NO_(x) and/or SO_(x).

[0018] The first and second temperature sensors and the NO_(x) sensortransmit their readings to a controller which transmits signals toproportional valve means located in the exhaust outlet downstream of thefirst temperature sensor. In response to the signals received from thecontroller, the proportional valve divides the exhaust gas stream fromthe engine exhaust manifold into two streams, one of which is sent to afirst conduit and the other is sent through a second conduit into a heatexchanger. Any stream exiting the heat exchanger is then blended withthe non-heat exchanged stream in the first conduit downstream of theproportional valve means.

[0019] The amount of the exhaust gas stream in the first conduit willvary from that in the second conduit such that (a) the temperature ofthe exhaust gas stream entering the catalytic converter is maintained inthe range of about 300 to about 600° C., preferably 350 to 550° C.,during the lean mode operation of the engine, and (b) the temperature ofthe exhaust gas stream entering the catalytic converter is periodicallyallowed to rise above about 600° C. to the extent necessary to purge theNO_(x) trap of SO_(x) i.e., when purging of the trap is required, littleor none of the exhaust stream received from the exhaust manifold is sentto the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a block diagram of an engine and its exhaust systemwhich embodies the features of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention is directed to an apparatus and method forvehicle emissions control.

[0022] The apparatus for vehicle emission control comprises:

[0023] a. an exhaust conduit for receiving the exhaust gas stream fromthe exhaust manifold of an engine;

[0024] b. proportional valve means in fluid communication with theexhaust conduit for dividing the exhaust gas stream into a first streamand a second stream between a first conduit and a second conduit;

[0025] c. a heat exchanger in fluid communication with the secondconduit, said heat exchanger containing an outlet conduit in fluidcommunication with the first conduit downstream of the valve means;

[0026] d. an underfloor catalytic converter in fluid communication withthe first conduit downstream of the heat exchanger outlet conduit, saidcatalytic converter containing a multi-functional catalyst and an outletconduit;

[0027] e. a first temperature sensor located in the exhaust conduitupstream of the valve means and a second temperature sensor located inthe catalytic converter (preferably near the inlet to the catalyticconverter), said first and second temperature sensors having thecapability of transmitting temperature readings to a controller;

[0028] f. a NO_(x) sensor located in the outlet of the catalyticconverter for sensing NO_(x) breakthrough;

[0029] g. a controller in communication with the first temperaturesensor, the second temperature sensor, the NO_(x) sensor and the valvemeans, said controller being capable of (i) receiving temperaturereadings from the first and second temperature sensors and readings fromthe NO_(x) sensor and (ii) transmitting signals to the valve means inresponse to the readings such that the valve means thereby controls theamount of exhaust gas stream to be divided between the first conduit andthe second conduit.

[0030] Preferably, the apparatus ofthe present invention includes aclose-coupled catalytic converter located upstream of the proportionalvalve means. It is particularly preferred that the close-coupledcatalytic converter be located immediately adjacent to (or affixed to)the exhaust manifold of the vehicle's engine.

[0031] The close-coupled catalytic converter preferably comprises athree-way conversion (“TWC”) catalyst, e.g., one or more platinum groupmetals disposed on a high surface area, refractory oxide support (theTWC catalyst is typically present in the form of one or more “bricks”enclosed within an appropriate container). Useful platinum group metalsinclude platinum, palladium, rhodium and mixtures thereof. A useful highsurface area, refractory oxide support comprises gamma-alumina It ispreferred that the gamma-alumina be stabilized with zirconia, titania,an alkaline earth metal oxide, one or more rare earth metal oxides andmixtures thereof. Preferable alkaline earth metal oxides include baria,calcia, strontia and mixtures thereof. Preferable rare earth metaloxides comprise ceria, lanthana and mixtures thereof It is preferredthat the refractory oxide support be disposed on a monolithic carrier.Preferably, the monolithic carrier comprises a refractory ceramic ormetal honeycomb structure.

[0032] The underfloor catalytic converter contains a multi-functionalcatalyst, typically a TWC catalyst ofthe type described above in respectto the close-coupled catalytic carrier in conjunction with a NO_(x)trap. Typically, the multi-functional catalyst for the underfloorcatalytic converter is also present in the form of one or more “bricks”enclosed within an appropriate container. Suitable NO_(x) trapcomponents include one or more basic oxygenated compounds of one or moremetals selected from the group consisting of alkali metals and alkalineearth metals, such as lithium, sodium, potassium, cesium, magnesium,calcium, strontium, barium and mixtures thereof.

[0033] Optimum results in respect to NO_(x) abatement will be obtainedduring lean operation of the engine (i.e., when λ>1) when thetemperature of the exhaust gas stream entering the underfloor catalyticconverter is maintained in the range of about 300 to about 600° C.,preferably 350 to 550° C. The first and second temperature sensors(e.g., thermocouples) will measure the temperature of the exhaust gasstream emanating from the exhaust manifold and entering the underfloorcatalytic converter, respectively, and will transmit the temperaturereadings to the controller. In response to the temperature readings, thecontroller in turn will transmit signals to the proportional valve meansand cause the valve means to be fully closed, be fully open or bepartially open at any intermediate position between full closure andfull opening depending on the signals received from the controller. Forexample, if during the lean mode operation of the engine, thetemperature reading of the first sensor is between about 300 and about600° C., the valve means will be fully closed such that all oftheexhaust gas stream will be diverted to the first conduit. If during theoperation of the engine, the temperature reading of the first sensor is,e.g., 900° C. or higher, the valve means will be fully open such thatall ofthe exhaust gas stream will be diverted to the second conduit(which leads into the heat exchanger). If, during stoichiometricoperation of the engine, the temperature reading of the first sensor is,e.g., 700° C., the valve means will be partially open and divide theexhaust gas stream between the first and second conduit.

[0034] The second temperature sensor serves as an “override” to thecontroller to thereby “fine tune” the amount of opening and closing ofthe valve means and hence the amount of the exhaust gas stream notheat-exchanged (i.e., diverted to the first conduit) and the amount ofthe exhaust gas stream heat-exchanged (i.e., diverted to the secondconduit). The desired result is that during the lean mode operation ofthe engine, the temperature of the exhaust gas stream flowing into thecatalytic converter will be maintained in the range of about 300 toabout 600° C., preferably 350 to 550° C.

[0035] As mentioned above, the NO_(x) trap must periodically be purgedof NO_(x) and SO_(x). The temperature required for purging SO_(x) willbe somewhat greater than the desired maximum temperature of 600° C. forthe exhaust gas stream entering the catalytic converter during thelean-burn mode of engine operation. The NO_(x) sensor located in theoutlet of the catalytic converter will sense that permanent NO_(x)breakthrough is occurring and transmit its readings to the controller.When NO_(x) breakthrough is such that purging of the trap is required,the NO_(x) sensor's readings sent to the controller will cause thecontroller to send a signal to the proportional valve means to preventthe exhaust gas stream from entering the heat exchanger (through thesecond conduit), and instead send the hot exhaust gas stream to theunderfloor catalytic converter for such period of time as is required toeffectuate the purging of the trap.

[0036] The apparatus and method of the present invention will be betterunderstood by reference to the detailed description of the drawing whichis set forth below.

DETAILED DESCRIPTION OF THE DRAWING

[0037]FIG. 1 shows an internal combustion engine 11, for example adirect-injection petrol or diesel engine equipped with underfloorcatalytic converter 21. Exhaust conduit 13 connects the exhaust manifold12 of engine 11 to proportional valve means 31. In a preferableembodiment of the present invention, a close-coupled catalytic converter14 is attached to exhaust manifold 12 and exhaust conduit 13 is attachedto the outlet of catalytic converter 14. Exhaust conduit 13 containsfirst temperature sensor 40 located upstream of proportional valve means31.

[0038] First conduit 32 and second conduit 33 are present in fluidcommunication with proportional valve means 31. First conduit 32terminates, and is fluid communication with, underfloor catalyticconverter inlet conduit 22 which in turn is in fluid communication withunderfloor catalytic converter 21. The exhaust gas stream exitsunderfloor converter 21 and is vented to the atmosphere (typicallythough a muffler, which is not shown).

[0039] Second conduit 33 terminates, and is in fluid communication with,heat exchanger 51. Heat exchanger 51 is equipped with outlet conduit 52which terminates, and is in fluid communication with underfloorcatalytic converter inlet conduit 22. Underfloor catalytic converterinlet conduit 22 contains second temperature sensor 42 located inunderfloor catalytic converter 21. Heat exchanger 51 is preferably agas/air exchanger from a simplicity point of view since the air flowalong the shell of the heat exchanger as the vehicle is in motionprovides sufficient cooling capacity without the need for any additionalgaseous or liquid coolant.

[0040] Temperature sensors 40 and 42 may be identical or different fromone another. Suitably, temperature sensors 40 and 42 will consist ofcommercially available thermocouples which will provide accuratetemperature measurements over the temperature range of about 300° C. toabout 1200° C. Temperature sensors 40 and 42 provide signals toelectronic engine controller (“EEC”) 61 over conductors 41 and 43respectively. Controllers such as EEC 61 are well known in the prior art(e.g., see U.S. Pat. 5,722,236) and typically comprises a microcomputerincluding input/output (“I/O”) ports, a central processing unit (“CPU”),read-only memory (“ROM”) for storing control programs, random accessmemory (“RAM”) for temporary dat storage and keep-alive memory (“KAM”)for storing learned values.

[0041] EEC 61 is in electronic communication with proportional valvemeans 31 by means of feedback conductor 62. Proportional valve means 31may be of any of the various commercially available types which arecapable of receiving electronic signals from a controller such as EEC 61and cause the valve to fully close, fully open or be open in anyintermediate position between fully closed or fully open

[0042] NO_(x) sensor 44, for sensing NO_(x) breakthrough and determiningwhen the trap must be purged to remove SO_(x), is located in the outletof underfloor catalytic converter 61 and transmits its readings ofNO_(x) and SO_(x) levels to EEC 61 via conductor 45.

[0043] In order to obtain very accurate control over the temperature ofthe exhaust gas, it is preferred that additional temperature sensors bepresent in the apparatus of the invention. Such additional sensors areshown in FIG. 1 as 46 and 48 which are located in the heat exchangeroutlet conduit 52 and second conduit 32, respectively. Additionalsensors 46 and 48 communicate with EEC 61 by means of conductors 47 and49, respectively. Additional temperature sensors may be provided asdesired and sensors 46 and 48 may be located at other positions in theapparatus as required for fine-tuning the opening and closure of theproportional valve means.

What is claimed is:
 1. An apparatus for vehicle emission controlcomprising: (a) an exhaust conduit for receiving the exhaust gas streamfrom the exhaust manifold of an engine; (b) proportional valve means influid communication with the exhaust conduit for dividing the exhaustgas stream into a first stream and a second stream between a firstconduit and a second conduit; (c) a heat exchanger in fluidcommunication with the second conduit, said heat exchanger containing anoutlet conduit in fluid communication with the first conduit downstreamof the valve means; (d) an underfloor catalytic converter in fluidcommunication with the first conduit downstream of the heat exchangeroutlet conduit, said catalytic converter containing a multi-functionalcatalyst and an outlet conduit; (e) a first temperature sensor locatedin the exhaust conduit upstream of the valve means and a secondtemperature sensor located in the catalytic converter, said first andsecond temperature sensors having the capability of transmittingtemperature readings to a controller; (f) a NO_(x) sensor located in thecatalytic converter outlet conduit for sensing when NO_(x) breakthroughis occurring; and (g) a controller in communication with the firsttemperature sensor, the second temperature sensor, the NO_(x) sensor andthe valve means, said controller being capable of (i) receiving readingsfrom the first and second sensors and the NO_(x) sensor and (ii)transmitting signals to the valve means in response to such readingssuch that the valve means thereby controls the amount of exhaust gasstream to be divided between the first conduit and the second conduit.2. The apparatus of claim 1 further comprising a close-coupled catalyticconverter located upstream of the valve means.
 3. The apparatus of claim2 wherein the close-coupled catalytic converter is located immediatelyadjacent to the exhaust manifold.
 4. The apparatus of claim 2 whereinthe close-coupled catalytic converter comprises a three-way conversioncatalyst.
 5. The apparatus of claim 4 wherein the three-way conversioncatalyst comprises one or more platinum group metals disposed on a highsurface area, refractory oxide support.
 6. The apparatus of claim 5wherein the platinum group metals are selected from the group consistingof platinum, palladium, rhodium and mixtures thereof.
 7. The apparatusof claim 5 wherein the refractory oxide support comprises gamma-alumina.8. The apparatus of claim 7 wherein the gamma-alumina is stabilized withzirconia, titania, an alkaline earth metal oxide, one or more rare earthmetal oxides and mixtures thereof.
 9. The apparatus of claim 8 whereinthe alkaline earth metal oxide is selected from the group consisting ofbaria, calcia, strontia and mixtures thereof.
 10. The apparatus of claim8 wherein the rare earth metal oxides are selected from the groupconsisting of ceria, lanthana and mixtures thereof.
 11. The apparatus ofclaim 5 wherein the support is disposed on a monolithic carrier.
 12. Theapparatus of claim 11 wherein the monolithic carrier comprises arefractory ceramic or metal honeycomb structure.
 13. The apparatus ofclaim 1 wherein the multi-functional catalyst comprises a three-wayconversion catalyst and a NO_(x) trap.
 14. The apparatus of claim 13wherein the three-way conversion catalyst comprises one or more platinumgroup metals disposed on a high surface area, refractory oxide support.15. The apparatus of claim 14 wherein the platinum group metals areselected from the group consisting of platinum, palladium, rhodium andmixtures thereof.
 16. The apparatus of claim 14 wherein the supportcomprises gamma-alumina.
 17. The apparatus of claim 16 wherein thegamma-alumina is stabilized with zirconia, titania, an alkaline earthmetal oxide, one or more rare earth metal oxides and mixtures thereof.18. The apparatus of claim 17 wherein the alkaline earth metal oxide isselected from the group consisting of baria, calcia, strontia andmixtures thereof.
 19. The apparatus of claim 17 wherein the rare earthmetal oxides are selected from the group consisting of ceria, lanthanaand mixtures thereof.
 20. The apparatus of claim 14 wherein the supportis disposed on a monolithic carrier.
 21. The apparatus of claim 20wherein the monolithic carrier comprises a refractory ceramic or metalhoneycomb structure.
 22. The apparatus of claim 13 wherein the NO_(x)trap comprises one or more basic oxygenated compounds of one or moremetals selected from the group consisting of alkali metals and alkalineearth metals.
 23. The apparatus of claim 22 wherein the NO_(x) trapcomprises one or more of basic oxygenated compounds of metals selectedfrom the group consisting of lithium, sodium, potassium, cesium,magnesium, calcium, strontium, barium and mixtures thereof.
 24. A systemfor vehicle emission control comprising: (b) an engine which generatesan exhaust gas stream and emits such stream through an exhaust manifold;(c) an exhaust conduit for receiving the gas stream from the exhaustmanifold; (d) proportional valve means in fluid communication with theexhaust conduit for dividing the gas stream into a first stream and asecond stream between a first conduit and a second conduit; (d) a heatexchanger in fluid communication with the second conduit, said heatexchanger containing an outlet conduit in fluid communication with thefirst conduit downstream of the valve means; (e) an underfloor catalyticconverter in fluid communication with the first conduit downstream ofthe heat exchanger outlet conduit, said catalytic converter containing amulti-functional catalyst and an outlet conduit; (f) a first temperaturesensor located in the exhaust conduit upstream of the valve means and asecond temperature sensor located in the catalytic converter, said firstand second temperature sensors having the capability of transmittingtemperature readings to a controller; (g) a NO_(x) sensor located in thecatalytic converter outlet conduit for sensing when NO_(x) breakthroughis occurring; and (h) a controller in communication with the firsttemperature sensor, the second temperature sensor, the NO_(x) sensor andthe valve means, said controller being capable of (i) receiving readingsfrom the first and second sensors and the NO_(x) sensor and (ii)transmitting signals to the valve means in response to such readingssuch that the valve means thereby controls the amount of exhaust gasstream to be divided between the first conduit and the second conduit.25. The system of claim 24 further comprising a close-coupled catalyticconverter located upstream of the valve means.
 26. The system of claim25 wherein the close-coupled catalytic converter is located immediatelyadjacent to the exhaust manifold.
 27. The system of claim 25 wherein theclose-coupled catalytic converter comprises a three-way conversioncatalyst.
 28. The system of claim 27 wherein the three-way conversioncatalyst comprises one or more platinum group metals disposed on a highsurface area, refractory oxide support.
 29. The system of claim 28wherein the platinum group metals are selected from the group consistingof platinum, palladium, rhodium and mixtures thereof.
 30. The system ofclaim 28 wherein the refractory oxide support comprises gamma-alumina.31. The system of claim 30 wherein the gamma-alumina is stabilized withzirconia, titania, an alkaline earth metal oxide, one or more rare earthmetal oxides and mixtures thereof.
 32. The system of claim 31 whereinthe alkaline earth metal oxide is selected from the group consisting ofbaria, calcia, strontia and mixtures thereof.
 33. The system of claim 31wherein the rare earth metal oxides are selected from the groupconsisting of ceria, lanthana and mixtures thereof.
 34. The system ofclaim 28 wherein the support is disposed on a monolithic carrier. 35.The system of claim 34 wherein the monolithic carrier comprises arefractory ceramic or metal honeycomb structure.
 36. The system of claim24 wherein the multi-functional catalyst comprises a three-wayconversion catalyst and a NO_(x) trap.
 37. The apparatus of claim 36wherein the three-way conversion catalyst comprises one or more platinumgroup metals disposed on a high surface area, refractory oxide support.38. The system of claim 37 wherein the platinum group metals areselected from the group consisting of platinum, palladium, rhodium andmixtures thereof.
 39. The system of claim 37 wherein the supportcomprises gamma-alumina.
 40. The system of claim 39 wherein thegamma-alumina is stabilized with zirconia, titania, an alkaline earthmetal oxide, one or more rare earth metal oxides and mixtures thereof.41. The system of claim 40 wherein the alkaline earth metal oxide isselected from the group consisting of baria, calcia, strontia andmixtures thereof.
 42. The system of claim 40 wherein the rare earthmetal oxides are selected from the group consisting of ceria, lanthanaand mixtures thereof.
 43. The system of claim 37 wherein the support isdisposed on a monolithic carrier.
 44. The system of claim 43 wherein themonolithic carrier comprises a refractory ceramic or metal honeycombstructure.
 45. The system of claim 24 wherein the NO_(x) trap comprisesone or more basic oxygenated compounds of one or more metals selectedfrom the group consisting of alkali metals and alkaline earth metals.46. The system of claim 45 wherein the NO_(x) trap comprises one or moreof basic oxygenated compounds of metals selected from the groupconsisting of lithium, sodium, potassium, cesium, magnesium, calcium,strontium, barium and mixtures thereof.
 47. A method for controllingvehicle emissions which comprises: (a) receiving an exhaust gas streamfrom the exhaust manifold of an engine through an exhaust conduit; (b)providing proportional valve means in fluid communication with theexhaust conduit for dividing the gas stream into a first stream and asecond stream; (c) flowing the first stream into a first conduit influid communication with the valve means and/or flowing the secondstream into a second conduit in fluid communication with the valvemeans; (d) providing a heat exchanger in fluid communication with thesecond conduit, said heat exchanger containing an outlet conduit influid communication with the first conduit downstream of the valvemeans; (e) providing an underfloor catalytic converter in fluidcommunication with the first conduit downstream of the heat exchangeroutlet conduit, said catalytic converter containing a multi-functionalcatalyst and an outlet conduit; (f) providing a first temperature sensorlocated in the exhaust gas conduit upstream of the valve means and asecond temperature sensor located in the catalytic converter, said firstand second temperature sensors having the capability of transmittingtemperature readings to a controller; (g) providing a NO_(x) sensorlocated in the catalytic converter outlet conduit for sensing whenNO_(x) breakthrough is occurring; (h) providing a controller incommunication with the first temperature sensor, the second temperaturesensor, the NO_(x) sensor and the valve means, said controller beingcapable of (1) receiving readings from the first and second sensors andthe NO_(x) sensor and (2) transmitting signals to the valve means; and(i) operating the valve means in response to the signals received fromthe controller to thereby divide the exhaust gas stream between thefirst conduit and the second conduit in amounts such that (1) thetemperature of the exhaust gas stream flowing into the underfloorcatalytic converter is maintained in the range of about 300 to about600° C. during lean mode operation of the engine and (2) themulti-functional catalyst is periodically purged of NO_(x) and/orSO_(x).
 48. The method of claim 47 wherein the temperature of theexhaust gas stream flowing into the underfloor catalytic converter ismaintained in the range of 350 to 550° C.
 49. The method of claim 48further comprising providing a close-coupled catalytic converter locatedupstream of the valve means.
 50. The method of claim 49 wherein theclose-coupled catalyst unit is located immediately adjacent to theexhaust manifold.
 51. The method of claim 50 wherein the close-coupledcatalytic converter comprises a three-way conversion catalyst.
 52. Themethod of claim 51 wherein the three-way conversion catalyst comprisesone or more platinum group metals disposed on a high surface area,refractory oxide support.
 53. The method of claim 52 wherein theplatinum group metals are selected from the group consisting ofplatinum, palladium, rhodium and mixtures thereof.
 54. The method ofclaim 52 wherein the support comprises gamma-alumina.
 55. The method ofclaim 54 wherein the gamma-alumina is stabilized with zirconia, titania,an alkaline earth metal oxide, one or more rare earth metal oxides andmixtures thereof.
 56. The method of claim 55 wherein the alkaline earthmetal oxide is selected from the group consisting of baria, calcia,strontia and mixtures thereof.
 57. The method of claim 55 wherein therare earth metal oxides are selected from the group consisting of ceria,lanthana and mixtures thereof.
 58. The method of claim 52 wherein thesupport is disposed on a monolithic carrier.
 59. The method of claim 58wherein the monolithic carrier comprises a refractory ceramic or metalhoneycomb structure.
 60. The method of claim 47 wherein themulti-functional catalyst comprises a three-way conversion catalyst anda NO_(x) trap.
 61. The method of claim 60 wherein the three-wayconversion catalyst comprises one or more platinum group metals disposedon a high surface area, refractory oxide support.
 62. The method ofclaim 61 wherein the platinum group metals are selected from the groupconsisting of platinum, palladium, rhodium and mixtures thereof.
 63. Themethod of claim 61 wherein the support comprises gamma-alumina.
 64. Themethod of claim 63 wherein the gamma-alumina is stabilized withzirconia, titania, an alkaline earth metal oxide, one or more rare earthmetal oxides and mixtures thereof.
 65. The method of claim 64 whereinthe alkaline earth metal oxide is selected from the group consisting ofbaria, calcia, strontia and mixtures thereof.
 66. The method of claim 64wherein the rare earth metal oxides are selected from the groupconsisting of ceria, lanthana and mixtures thereof.
 67. The method ofclaim 61 wherein the support is disposed on a monolithic carrier. 68.The method of claim 67 wherein the monolithic carrier comprises arefractory ceramic or metal honeycomb structure.
 69. The method of claim47 wherein the NO_(x) trap comprises one or more basic oxygenatedcompounds of one or more metals selected from the group consisting ofalkali metals and alkaline earth metals.
 70. The method of claim 69wherein the NO_(x) trap comprises one or more of basic oxygenatedcompounds of metals selected from the group consisting of lithium,sodium, potassium, cesium, magnesium, calcium, strontium, barium andmixtures thereof.
 71. The method of claim 47 wherein: (a) the NO_(x)sensor senses that NO_(x) breakthrough is occurring and transmits areading of the NO_(x) breakthrough to the controller, (b) the controllerreceiving such reading transmits a signal to the valve means; and (c) inresponse to such signal, the valve means causes the exhaust gas streamto be divided in amounts between the first conduit and the secondconduit such that the temperature of the exhaust gas stream entering theunderfloor catalytic converter is in the range of about 600° C. to about750° C. such that the multi-functional catalyst is purged of SO_(x).